Synthetic resins are currently used to produce many different products. Among them are bottles, cans, drums, tanks, and toys.
One method for processing synthetic resins is the "extrusionblow method". In such method, the processing occurs while the workpiece is in a heated state. Typically, a tube is pressed out of a blowing head and subsequently divided into preform blanks which are transferred, while in a plastically deformable state, into a blow mold where they are blown into completed hollow bodies.
Alternatively, the "reheat process" may be used to produce an end product from a preform blank. In this process preform blanks which have been produced by direct injection molding are cooled and then placed in storage. Subsequently, they are removed from storage and reheated prior to blow mold processing. The present invention is directed to a novel reheat process.
The reheat process has many advantages over the extrusion-blow method. The structural characteristics of the preform blanks created during the injection process are preserved by the rapid cooling which occurs following the injection molding of the work material. These beneficial structural conditions can be further improved by reheating, mechanical working and blow molding. The resulting structural properties improve the end product's droprupture resistance, cold-rupture resistance as well as its visual transparency.
Another advantage of using the reheat process is when hollow bodies are produced that have filling and emptying openings, such as bottles, flasks, containers, canisters or the like, the area to be shaped for the sealing can be precisely shaped during the initial injection molding process and does not need to be again shaped during the blow molding process. This is particularly important when producing threaded surfaces. As a result of this feature, the blow molding time can be reduced because the neck area contains the largest cross section of the hollow body, and if shaped in a blow mold apparatus, it requires a significantly longer period of time to cool than the rest of the hollow body.
In the reheat method, preformed blanks which have been typically formed by injection molding are removed from storage and reheated to the desired blow molding temperature. This is normally accomplished by conveying the blanks along an infrared radiating system which heats the preform blanks to a uniform temperature including the temperature of the material within the transverse cross sections.
A major challenge involves the heating of the preform blank uniformly throughout its entire cross section. The preform blank is continuously or intermittently heated while being conveyed and at the same time while being rotated about its longitudinal axis. The preform blank is, however, only exposed to radiant heat on its outside surface and a temperature gradient is produced through its cross section. When the preform blank is blow molded axial stretching typically occurs. If the temperature through the material of the preform blank is not substantially the same, the end result is the production of a hollow body having certain undesirable qualities. The inside surface of the preform blank is structurally different than the outside which was formed at a greater temperature than the inside. When the inner portions of the preform blank are deformed during the blow molding process while the temperature of such portions is relatively low, stress distortions are usually created therein which have a negative influence on the strength and the appearance of the resulting hollow body.
According to European Patent No. 387 737 A1, a satisfactory uniform heating of a preform blank can be provided within a relatively short period of time. According to that method, the preform blanks are heating to avoid recrystallization at a temperature that is lower than the blow molding temperature. They are subsequently cooled, and then reheated to a temperature slightly above the blow molding temperature. Thereafter, the temperature of the preform blanks is allowed to equalize or, in any event, to decrease slightly prior to blow molding.
This method is not suitable, however, for certain desirable processes. Not all plastic materials of the group of partly crystalline polymers, respond favorably to the method. An outstanding representative from the group of partly crystalline polymers, is polypropylene. Polypropylene has many desirable qualities. These include: a very high rupture resistance, high drop values, excellent transparency and a thirty percent lower specific weight relative to other synthetic resins. (e.g. polyethylene terephthalate). Furthermore, polypropylene is available at approximately one-half the cost of other synthetic resins. Yet further, compounds which are made of polypropylene are suitable for filling with hot products. Still further, the polypropylene containers as well as their contents can be sterilized after they have been filled. This can be important, for example, for medicines and products such as blood plasma.
Partly crystalline synthetic resins such as polypropylene are extremely difficult to process due to their narrow "processing window". The "processing window" is severely restricted by the limited temperature range in which partly crystalline synthetic resins can be suitably processed to produce the favorable properties described above. Polypropylene, for example, has a very specific optimum processing temperature which must be maintained within a range of .+-.1 degree Celsius.
Another reason for the difficulty in processing polypropylene relates to its composition. Polypropylene does not contain free hydrogen molecules. When materials having free hydrogen molecules are heated, they tend to vibrate which assists in the attainment of a uniform temperature distribution within such materials. In contrast, when processing polypropylene, nearly all of the total heat must be applied along the length and through the cross section of the preform blanks by conduction, as such, without the benefit conferred by the free hydrogen molecules.
A method which has been used to heat polypropylene preform blanks in a uniform fashion is known as the "Hercules Method". With this method, the polypropylene preform blanks are exposed to slow, careful heating during a long period of time and over a relatively long distance of their movement on a conveyor. This method attempts to arrive at the desired uniform temperature in cross section of the preform blanks by continuously heating the blanks over a relatively long time duration. Typically, in the Hercules Method the preform blanks are heated for long as a thirty minutes time period. During most of this period, the temperature of the material throughout the cross-section of the preform blanks is being equalized.
The Hercules Method has serious drawbacks. When molding or heating work products, periods of shutdown are inevitable. Shutdown times can occur both intentionally as well as for reasons which are beyond of the control of the operator. When shutdowns occur with the Hercules Method, preform blanks which remain in the oven are often overly heated whereby they become unusable rejects.
For the foregoing reasons, there is a need for a reheat method which can be successfully used to process synthetic resins including, in particular, partly crystalline synthetic resins such as polypropylene. But, such reheat method should also be capable of processing preform blanks of multiple layers of different materials which can contain both partly crystalline as well as amorphous materials. Furthermore, it is advantageous for the reheat method to have the required treatment time substantially reduced-preferably to only a very few minutes, for example, a five minute total processing time would be ideal.